Electronic devices such as portable computers include a circuit board and electronic parts mounted on the circuit board. The electronic parts include, for example, LSI. The circuit board is provided with a number of signal lines which form a circuit.
In the case where signals to be input to the electronic parts and signals to be output from the electronic parts are of high frequencies, a signal line through which these signals pass has a resistance and an inductance. Therefore, characteristic impedance is created in the signal line.
In the case where the impedance of an electronic part and the characteristic impedance of the signal line are different from each other, the signal reflects at the junction between the electronic part and the signal line, thereby generating noise. In order to prevent the generation of noise, the characteristic impedance of the signal line and the impedance of the electronic part need to be matched with each other.
The characteristic impedance of the signal line is determined by the width of the signal line and the thickness of the insulating layer between the reference layer and the signal line. The reference layer is, for example, a ground layer or a power layer. As the width of the signal line is narrowed, the characteristic impedance increases. As the thickness of the insulating layer is reduced, the characteristic impedance decreases. In accordance with the impedance value of the electronic part, the width of the signal line and the thickness of the insulating layer are determined. In this manner, the impedance of the electronic part and the characteristic impedance of the signal line are matched with each other.
Incidentally, with regard to electronic devices such as portable computers, there is a demand for making the devices slim to be easily portable. In such electronic devices, the circuit boards are designed to be thin, and accordingly the insulating films are thin. When the insulation films are thin, the characteristic impedance of the signal line is low. Here, in order to match the impedance of the electronic part and the characteristic impedance of the signal line with each other, the characteristic impedance should be increased by reducing the width of the signal line.
However, it is technical difficult to narrow the signal line further once the width is reduced to a certain level. As a solution to this, in the circuit board disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-141990, a plurality of perforations are made in the ground layer. The perforated ground layer formed into a mesh state has a smaller area in which the signal lines and the reference layer face each other. With this structure, even in the case of a signal having a large width, the impedance of the electronic part and the characteristic impedance of the signal line can be matched with each other.
Further, when a signal passes through a signal line, a return current flows in the ground layer in the direction opposite to the signal passing direction. In the circuit board disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-141990, the perforations are made in the ground layer. With this structure, the return current detours the perforations when flowing the mesh ground layer.
As described above, the return current travels a longer distance than that of the signal passing through the signal line. Therefore, the return current delays with respect to the signal. When the return current delays, the signal recognition error may occur. Thus, circuit boards thinned and impedance-matched with electronic parts are susceptible to improvement.